Distance measuring apparatus having a digital output circuit



April 27, 1965 R. J. HEPPE ETAL DISTANCE MEASURING APPARATUS HAVING A DIGITAL OUTPUT CIRCUIT Filed June 9. 1960 12 Sheets-Sheet 2 m u m & wosmx m IN VEN TORS.

ROGER 7' J. HEPPE RAY/1 N0 L. PICK/{0L 72 BY 5 .4 TTORNEY April 27, 1965 R. J. HEPPE ETAL DISTANCE MEASURING APPARATUS HAVING A DIGITAL OUTPUT CIRCUIT Filed June 9, 1960 12 Sheets-Sheet 4 E E E A B c TIME INVENTORS. ROBERT J. HE'PPE RAYMOND L. P/CKHOL 7'2 ATTORNEY lllll ll ll.

April 27, 1965 R. J. HEPPE EIAL 3,180,205

DISTANCE MEASURING APPARATUS HAVING A DIGITAL OUTPUT CIRCUIT Filed June 9, 1960 12 Sheets-Sheet 5 mmvms.

ATTORNEY I mm A. QaowG wwwwwwL a a1 R .8 3% V M ROBERT \l. HEPPE By RAYZOIVO 1.,P/Ckf/0LT2 April 27, 1965 R. J. HEPPE EI'AL 3,180,205

DISTANCE MEASURING APPARATUS HAVING A DIGITAL OUTPUT CIRCUIT Filed June 9. 1960 12 Sheets-Sheet 7 eeA SURFA c 814 INVENTORS. ROBERT J. HEPP BY nAYMoAm L. P/Ckl/OLTZ A T TORNEY April 27, 1965 R. J. HEPPE EI'AL DISTANCE MEASURING APPARATUS HAVING A DIGITAL OUTPUT CIRCUIT Filed June 9. 1960 12 Sheets-Sheet 9 xwwiLWni.

.6 u v. R R6 L E 5 m mm mi N 5/,NA RI. TPK R 1/ AC 0 PC r m 5 V T w W T IJ.L A 4 m mm. MY v. m N/ 5F Y 8 M I o A Z I w A n E 9 w M A a O M+ 4 8 7 \.1\& IA k April 1955 R. J. HEPPE ETAL 3,180,205

DISTANCE MEASURING APPARATUS HAVING A DIGITAL OUTPUT CIRCUIT Filed June 9, 1960 12 Sheets-Sheet 10 "09 182 gU'i i 7:37 61w. 7 84C //46c z777\ 50 AMA/v. A /'/38 /,/46 7346 p 0 GI INVENTORS,

5 ROBL-RT u, HEPPE 74/ 4/A RAYMONO L. P/CKHOLTZ ATTORNEY face, because they find nothing to 3,1$0,205 DISTANCE MEASURING APPATUS HAVING A DIGITAL OUTPUT CIRCUIT I H Robert J. Heppe, North Caldwell, and Raymond L. Pickholtz, West Englewood, NJ assignors to International Telephone and Telegraph Corporation, Nutley, N.J., a

corporation of Maryland Fiied June 9, 1960, Ser. No. 34,946

7 Claims. (Cl. 88-1) Our invention relates to distance measuring apparatus and is particularly concerned with means utilizing active radiation from the apparatus in order to create data for the measurement of the distance from the apparatus to a surface. i

. In the prior art, many techniques are known for measuring distance using both active and passive means. Some of the principles have been embodied in distance measuring devices, such as altimeters, range finding devices and interferometers; Some types of altimeters have utilized electromagnetic radiation transmitted from the aircraft and reflected from theground to measure the altitude or distance from the aircraft to the ground. 7 Such altimeters United States Patent 3,180,205 Patented Apr. 2?, 1965 An additional object of this invention is to provide a system for measuring distance which can be readily adaptablev to provide various degrees of precision in measurement.

It is an object of this invention to provide an active,

- absolute altimeter.

It is a feature of this invention to' project beams of radiation of one or more wavelengths'upon the surface, the distance-to which is to be measured. The reflection from the intersection of this beam or beams with this a surface is sensed by a fixed or a movable receiver. As

have had many undesirable features. For example, the 7 radiation from the transmitter of such a device. employs Waveforms of a special nature and it is necessary to precisely synchronize the receiver withthe transmitter. Also the precision of measurement of such devices is not readily varied. To obtain precise measurement of distance, such radar-type altimeters must measure time intervals with great precision involving measurements on the order of a receiver, and to providethe proper processing of the radiation received by the receiver. In addition, this type of apparatus is not suitable for measuring relatively short distances on the order of less than 10 feet, forexample.

There are other classes of devices which utilize principles of interferometry. These devices measure distance in terms of the wavelength of the radiation by examining interference patterns which are produced on the surface the distance to which is to be measured. These techniques produce very great accuracy. However, the apparatus is normally rather bulky and fragile and the degree of precisionproduced is not'ordinarily required for most applications. The associated electronic circuitry that such equipment requires is also quite complex. Devices utilizing interferometric techniques are suitable especially for matic range finders. These are passive devices which uti- I lize the radiation from objects which is produced by the objects own temperature or by reflected radiation from the sun or other sources of illumination. Such devices I suffer from the defect that to measure the distance it is required that there be an object or target that stands out I from the surrounding background. These devices are incapable of measuring distance to a relatively uniform suract as a target on which to fix their operation. 1

It is an object of this invention to provide an active device which measures the absolute distance from the device to a surface. 'i j It is another object of this invention to provide a distance measuring apparatus which is readily portable or movable.

It is a further object of this invention to provide a distance measuring apparatus which can be adapted tov measure a wide range of distances with a high degree of accuracy. T

the invention;

the distance from the apparatus to the surface changes, changes take place in the size and/ or, relative location of the intersection of the beams with the surface so that for any particular distance to the surface, the output of the receiver or scanner is distinctly characterized.

It is an additional feature of this invention to project two intersecting beams ofrradiation upon the surface to which the distance is to be measured. These beams of radiation are so trigonometrically arranged with respect to each other and two receivers that for any particular distance from the apparatus to the surface, there is a particular separation between the intersections of the'two beams with the surface. The two receivers or scanners optically scan the two spots of radiation on the surface and determine the separation between these two spots or intersections. The associated circuitry converts this separation into a signal with both sense and magnitude indicating the distance from a reference distance to the surface.

The above-mentioned and other features and objects of this invention will become more apparent by reference to the following description taken in conjunction with the accompanying drawings, in which: a

,' bodiments of FIGS. 1, 2, and 3 may be utilized in a closed loop servo system to control distance,

FIG. 6a is a diagram illustrating a second embodiment of the invention;

FIG. 6b shows the relationship between'the geometry of FIG. 6a and the pulses which are derived from this geometry;

FIG. 7a shows a schematic and block embodiment of FIG. 6;

FIG. 7b illustrates one of the circuits of FIG. 7a,

FIG '8 is a series of timing diagrams showing the waveforms at various points in the block diagram of FIG. 7a in relation to each other; a

FIG. 9 is an idealized plot of the characteristics of th average value detector shoWn-inFIGS. 7a and 7b.

FIG. 10 is a diagram illustrating diagram for the FIGQ 11 is a trigonometric diagram useful in describing the operation of the embodiment of FIG. 10;

FIG..12is'ablock diagram illustrating the operation of I the embodiment of FIG. 10; FIG. 13 is a perspective'drawing showing some of the details of construction of the scanner shown in FIG.'12;

a third'embodiment of FIG. 16 shows an alternative arrangement of parts of the scanner shown in FIGS. 12, 13 and 14.

FIG. 17 is a side view showing a fourth embodiment of the present invention;

FIGS. 18a, b and c illustrate the field of vision of the scanner shown in FIG. 17;

FIG. 19 is a block diagram of the embodiment shown in FIGS. 17 and 18;

FIG. 20 is a side view illustrating a fifth embodiment of the invention; I

FIG. 21a is a view of the images produced on the face of the scanner used in the embodiment of FIG. 20;

FIGS. 21b, 21c and 21d show the resulting pulse outputs from the scanner of FIG. 21a.

FIG. 22 is a block diagram illustrating the arrangement of electronic components for the embodiment shown in FIG. 20;

FIG. 23 is an alternative block diagram showing the arrangement of components for use with the embodiment of FIG. 20.

Referring now to FIG. l the apparatus is shown mounted on a support 1, and includes two sources of electromagnetic radiation, elements 2 and 3. The two sources of radiation, 2 and 3, may emit visible light, ultraviolet radiation or infrared radiation. In some applications, in the control of machine tools, for example, visible light may be desirable for maintenance and alignment purposes. In other applications, 'for example, in an altimeter for use on an aircraft, it might be desirable to use ultraviolet or infrared radiation, to avoid detection from the ground, but the principles involved are the same whatever particular wavelength (A) is used. A lens system 4 is mounted on the front of source 2 and may consist of one or morelenses for focusing and directing the beam 5 emitted from source 2. The lens system 4 may consist of one or more lenses'utilizing Well known optical principles. For the instant embodiment, there are only two requirements placed uponthe nature of beam 5. First, beam 5 must be radiation of a definite predetermined wavelength (A1). This wavelength Al of the radiation emitted by source .2 can be anywhere within the range mentioned, visible light, ultraviolet or infrared. The second requirement placed on beam 5 is that the leading edge 6 of beam 5 must be fairly well defined. Such a beam with a well defined edge can be obtained by a simple lens system such as a slit or aperture in a plate, for example. Thu lens system 4 might simply be a slit in a plate. The necessity for the sharp leading edge 6 will be explained below. Likewise, the second source 3 has a lens system 7 for focusing and directing the beam 8 emitted by source 3. Again, there are only two requirements placed upon the'beam" 8 from I d taken as a side elevational view and the actual disposition of the system in space makes no difference; it is only the relative location of apparatus and surface 12A which is important. For convenience, we will refer to the reference direction as being horizontal and parallel to the mounting plane 13 on which projectors 10 and 11 are mounted. But it is to be understood that mounting plane 13 may have many other actual dispositions in space. Thus, projector 10 makes the predetermined angle 13 from mounting plane 13. Likewise, rojector 11 is so mounted with relation to mounting plane 13, that the beam 8 from projector 11 forms the angle C with plane 13. Both beams 5 and 8 intersect the surface 12A in definite intersection areas; beam 5 intersects surface 12A in intersection area 14, and beam 8 from projector 11 intersects surface 12A in intersection area 15'. Beams 5 and 8 are directed at given angles B and C so that beams 5 and 8 intersect each other at intersection area 16. The two projectors 10 and 11 are separated by a distance D denoted in FIG. 1. The required relationships between distance D and angles B and C Will become apparent from the discussion below.

Also mounted on support 1 are two receivers, 17 and 18. Thus, receiver 17 is fitted with a lens system 19 for focusing and directing within receiver 17 the beam of radiation originating by reflection at 14-, the intersection projector 3. The first is that the leading edge 9 of beam 8 must be fairly well defined. The second'requirement for beam 8 is that it must be of a definite wavelength (A2) which is different from the wavelength (A1) of the beam 5. Thus, the beam 8 emitted by source 3 must be of a different wavelength A2 than the radiation in beam 5 emitted by source 2. Again, the Wavelength A2 used for beam 8 can be anywhere within the visible, infrared, or ultraviolet spectrum so long as it is distinguishable from the wavelength A1 used for beam 5.

Thus, the source of electromagnetic radiation 2 in the lens system 4 makes up a first transmitter or projector 10 of electromagnetic radiation producing a beam of radiation 5. Likewise, the source of electromagnetic radiation 3 along with lens system 7 make up a second transmitter or projector 11 producing a beam of radiation 8. The beams Sand 8 from projectors 10 and 11 are directed toward a surface 12a. Projector 10 is so located with respect to support 1 that projector 10 makes an angle B from the horizontal or reference direction. It should be emphasized that FIG. 1 can be taken as a top plan view of the embodiment or it can also be and filter 21 as the first scanner, 23.

of beam 5 and surface 12A. Likewise, receiver 18 is fitted with the lens system 20 for focusing and directing within receiver 18 the beam of radiation originating by reflection from the intersection 15 of beam 3 and surface 12A. In addition, receiver 17 is fitted with a filter 21 which allows only radiation of the wavelength (A1) by source 2 and projector 16 to enter into and act upon receiver 17. Likewise, receiver 18 is fitted with the filter 22 which only allows the wavelength A2 emitted by source 3 and projector 11 to enter into and act upon receiver 18. It is possible in certain types of construction to omit filters 21 and 22 as will be explained below. For convenience, we denote receiver 17, lens system 19 Likewise, for convenience, we denote receiver 18, lens system 20 and filter 22 as second scanner 24. Thus, the first scanner 23 is responsive only to radiation of Wavelength A1 produced by projector 10 and second scanner 24- is responsive only to radiation produced by projector 11 of wave length A2.

Both scanners 23 and 24 scan or observe the surface 12A in the direction shown in FIG. 1. This scanning can be accomplished by physical motion of the projector, or by electronic means. Both types of scanning will be described in detail later in this application.

, The operation of the embodiment of FIG. 1 can now be explained. The two beams of radiation 5 and 8 produce two spots, or targets, 14 and 15 which are the intersections of the beams 5 and 3, respectively, with the surface 12A. Both scanners begin to examine the surface 12A starting to the left of the FIGURE 1, and moving towards the right. This scanning action takes place many times per second at a convenient scanning rate. A typical electronic scanning rate might be one kilocycle per second, in other words, 1000 scanning cycles per second. As the scanners 23 and 24 begin to look at surface 12A directing their attention from the left of the figure to the right, as shown by the arrows, first one scanner will observe the reflections from one of the intersections and then at some time later the other scanner will observe and respond to the reflection from the other intersection. The distance to be measured'is the distance between the apparatus, mounted on plane 13, and the surface 12A. This distance is shown as X in FIG. 1. Assume that initially the distance X is that indicated as k which is still unknown. With distance X at value I2 as the scanners examine surface 12A, the following takes place. Scanners 23 and 24 will both receive radiation from spot or target 15, at the same in A2 as determined by source 3 in projector 11.

5. stant. But spot or target 15 is formed on surface 12A by the intersection of beam 8 originating from projector 11; and the radiation reflected from target area 15 towards scanners 23 and 24 still has the wavelength Thus, the radiation from spot 15 will reach scanners 23 and 24 at the same time. Also, both scanners will look at spot 15 at the same time. However, only the second scanner 24 will respond to this'radiation because filter 21 'on scanner 23 will prevent scanner 23 frornresponding in any way to the'spot 15. Thus, as scanners 23 and 24 encounter spot 15, an output will originate at that instant from the second scanner 24 only. As the scanners continue to examine the surface 12A moving from left to right, both scanners will next encounter radiation reflected from the spot or target 14. But spot or target 14 is created by beam from projector 19, and the radiation reflected from spot 14 has the wavelength A1 as determined by source 2 and projector 10. Now only the first scanner 23 will respond to target 14 because the filter 22 in the second scanner 24 will not permit the second scanner 24 to respond to radiation from spot 14. As both scanners, 23 and 24 examine spot 14 on surface 12A an output will originate only from the first scanner 23. 1111155101 a complete scanning cycle moving from left to right, there will first be an output from scanner 24. Then, at some time later, there will be an output from scanner. 23 .v The utilization means connected to the outputs of scanners 23 and'2-4 measures the time separation between the outputs from the two scanners, and provides an indication of altitude obtained from such time separation. If the scanning speed is uniform moving from left to right, therewill be a .direct proportion between the time separation of the pulses at the outputs of scanners 23 and 24 and the distance separation of targetsorspots 14 and 15 on the surface Assume that the surface 12A and the support 1 move two is given as I1 Thisdistance h is the distance corresponding to the point where thetwo spots intersect each other at 16. The distance h is that corresponding to the condition of beams 5 ancl'8 intersecting each' other,

forming at this one particular distance only one illuminated spot on surface 12A. Thus, the surface 12A has moved tothe position shown as 1213 by the broken line passing through the intersection of thetwo bearnsf Now, when the scanners scan their fleld of vision, each scan- .ner will encounter only one spot or target to which it the two scanners 23 and 24. This particular distance I1 It will'be noted forcan be used as a reference distance. X having values greater than h such as 11 separations between the spots 14 and 15 is'not unique. For each dis closer together so that the new fdistance separating these beam 5 fromthe first projector 10 will intersect surface 12C in a target spot shown as 140 and the beam 8 from projector 11 will intersect surface 12C in the spot denoted as 15C. As before, the scanners 23 and 24 will examine the surface 12C in unison, the direction of scan as before being from the left to the right. Both scanners 23 and 24 will first encounter spot 14C in their field of vision as they scan. The radiation reflected from spot 14C has the wavelength A1, as previously described. Thus, the scanner 24 will not respond to target spot 14C at all. However, scanner 23 will respond to target spot 14C because scanner 23 has beenarranged to respond only to radiation of wavelength A1 such as emitted by source 2 in projector 10. Thus, duringone scan cycle, as the scanners 23 and 24 examine their field of vision moving from the left to the right, scanner 23, with receiver 17 will encounter and respond to spot 140. At a later time, both scanners will'encounter spot or target 15C on surface 12C. However, only scanner 24 will'respond to reflection from spot 15C because filter 21 on scanner 23 will prevent scanner 23 from responding to radiation from spot 150. Thus the sequence of events will be this: first scanner 23 will produce an output; then at some later time, scanner 24, will produce an output. Reference to the discussion above when the distance was at a value h will show that previously an output was caused by radiation of wavelength A2 in spot 15 and then later an output was caused by radiation from spot 14 of wavelength A1. The occurrence of the two wavelengths has now been reversed, and now wavelength A1 causes the first output from receiver 17 by means of spot 14C and then later wavelength A2 and beam 8 causes an output from receiver 18, from reflections from spot 15C. Thus, the use of the two'ditferent wavelengths and two receivers each responsive to only one wavelength has produced a'system which will produce 'an indication of altitude which hasboth sense and magnitude.

Thus, if scanner 23 first responds to wavelength A1, then the distance from the support 1 to surface 12 must be less than the'value 11 Such a vame is h as has been just illustrated. On the other hand, if scanner 24 first responds to wavelength A2, then the distance from the support 1 to the surface 12A must be greater than the distance k Such a distance is h and it was used to extance greater than I1 there isa corresponding distance less than-I for example hiQwhere the very same distance and time separationniay occur between the two spots 14 and 15. But it will be shown that the scheme shown in FIG. 1 allows an unambiguous determination of which of the two ranges the distance X actually lies in, that greater than k or that less than 11 This can be done because the intersection of the two beams provides a point from which the sense of the signal received from the two scanners 23 and 24 can be determined. To illustrate, assume that the surface 12A moves even closer to support 1 to the new position indicated by the distance I1 and assumes the position 120. Now note that and closer together.

"no difference if surface 12A'is movingtransversely with plain the beginning of the discussion of the invention. When the outputs from the two scanners occur at exactly the same time, there is only one particular distance h which corresponds to this condition. The output from 'the two scanners are introduced to utilization means 25 IP1 the separation between spots 14C and 15C, for ex-,

ample, will also increase as'the distance grows smaller and smaller and support 1 and surface. 12A movecloser It should be noted that it makes respect to support 1 because the projectors 10 and 11 create virtual targets by their illumination of the surface and the location of these virtual targets 15 and 14 are fixed by the geometry of the arrangements of the apparatus itself. Transverse motion of the surface12A parallel to the direction of the scanning arrow will make no difference in the operation of the device. Motion is measured along the line perpendicular to plane 13, and surface 12A. FIG. 1 has been drawn for ease of illustration with angles B and C being approximately equal, but this is not necessary. Angle B need not-be equal to angle .to travel in straight lines in FIG. 1.

C and projectors 1i) and 11 do not have to be located along a horizontal line. One projector can be at a higher or larger distance from surface 12 than the other. In addition, it is not necessary that scanners 23 and 24 lie in the same plane with projectors 10 and 11. Scanners 23 and 24 can be set back in depth from the plane which contains the two projectors 10 and 11 and the two beams 5 and 8. All that is necessary is that the two beams 5 and 8 intersect at a common point at a predetermined distance from plane 13 and that the distance D between the projectors 10 and 11 and the angles 13 and C be known. This may change the variation in the scale corresponding between time separation of thev outputs of the scanners 23 and 24, and corresponding distance. The utilization means 25 can be readily arranged to accommodate such a different variation or change in scale.

FIG. 2 shows in some detail a block diagram utilizing the invention of FIG. 1 showing typical electronic circuitry to provide an indication of the distance X. It should be noted that the reflections in FIG. 1 are not strictly optical reflections; that is, radiation is assumed However, at the intersections of the beams with the surface 12A at and 14, the angle of incidence does not equal the angle of reflection of the radiation. This 'is because surface 12A it assumed to be of sufiicient roughness or coarseness to cause a scattering and diffusion of the incident radiation so that targets 14 and 15 can actually be viewed from any angle by the scanners 23 and 24. It would be extremely unusual to find any surface other than a mirror to be so smooth that the angle of incidence would equal the angle of reflection exactly so that spots 14 and 15 could only be viewed from one angle'. Under most conditions, scanners 23 and 24 will be able to observe spots 14 and 15 from any arbitrary angles that are convenient because of the diffusing and scattering action of surface 12 upon radiation incident in beams 5 and 8: In FIG. 2, projector 10 emitting a beam of radiation 5 which has wavelength A equal to A1 is shown in abbreviated form and projector 11,v as described in FIG. 1, is shown emitting a beam of radiation 8 which has wave length x equal to A2. Scanner 23 is composed of lens 19 and filter 21 as mentioned before, and an electronic scanning tube 17', such as a vidicon tube. Such tubes are well known in the art and are commercially available. Likewise, scanner 24 is composed of lens system 20, filter 22 and electronic scanning tube 18. Electronic scanning tubes 17 and 18 can be exactly the same types of tubes. The discussion here will be made assuming that 17 and 18' are vidicon type tubes. They are used in television systems as camera tubes. Image orthicon-type tube can also be used as electronic scanning tubes 17 or 18 since the operation will be exactly analogus to that of vidicon tubesl Both types of tubes are described in some detail on pages 981 through 986 of Electronic and Radio Engineering edited by Frederick E. Terman, published by McGraw-Hill 'Book Company, Inc., New York 1955.

The vidicon tube has a number of standard inputs and an output. We are concerned here only with one input,

namely, one of the two sweep inputs. This sweep input causes the electron beam within the tube to be successive: ly moved across the face of the tube. As it encounters light which is focussed upon the face of the tube, an output is produced at the output terminals of the tube. The sweep input to vidicon tube 17' has been indicated at terminal 2 6 and the sweep input terminal tovidicon tube 18 has been indicated at terminal 27. A normal vidicon tube has two sweep inputs, one for thevertical deflections and one for the horizontal deflections. The present system requires sweeping in only one direction which could be taken as either the horizontal or the vertical simply by rotating the vidicon tube. Therefore, vidicon tubes used in the present system could be somewhat simpler in con struction than the normal type vidicon tube effecting a possible saving in cost. The same sweep signal is used and instant in time.

connected in parallel to vidicon tubes 17' and 18 so that the two tubes are swept in synchronism. The output terminal of vidicon tube 17' is indicated at terminal 28 and the output of vidicon tube 18 is indicated at terminal 29. Thus, terminal 28 is the output of scanner 23 and terminal 29 is the output of scanner 24. The power supplies, focusing. controls, and so on have been omitted for simplicity. The two scanning tubes 17'- and 18' with their filters and lens systems, are arranged in close proximity to each other so that the field of vision of each scanner is essentially the same. The two tubes are also swept in unison so that each tube examines the same part of the field of vision as the other tube at the same instant in time. This establishes the required relationship between the outputs in time of the two tubesf Projector 10 which contains source 2 and emits beam of radiation 5 with Wavelength A1 has been disposed to show reflections from beam 5 arriving only at vidicon tube 17' of scanner 23. Actually reflections from beam 5 and projector 10 will arrive at both vidicon tubes but because of the filter arrangement which we have pointed out in FIG. 1, only vidicon tube 17 will respond to radiation of wavelength A1, so that for simplicity the drawing in FIG. 2 has been made in that manner. Likewise, the radiation from source 2 within projector 11 and emanated in beam 8 of wavelength A2 has been shown entering second scanner 24 and being focussed upon the face of vidicon tube 18'. Likewise, reflections from beam 8 will actually enter the scanner 23 and also be focussed upon the face of vidicon tube 17. But only scanner 24 will respond to radiation of wavelength A2 because of filter 21 as discussed above. Thus, the two scanners examine the surface 12A in unison looking at the same point on surface 12A at the same But at any given instant, only one of the two vidicon tubes 17' and 18' can respond with the exception of the one point where the two beams cross and intersect in point 16. Under that condition, both vidicon tubes will respond at the same instant. The lens systems 19 and 20 are so arranged that'the images of the spots 14- and 15 will be focussed respectively upon the surface of vidicon tubes 17' and 18'.

FIGURE 3 shows a series of timing diagrams. The lower case letters by each waveform in FIG. 3 correspond to the lower case letters shown in the block diagram of FIG. 2, so that the waveforms at each point in the block diagram of FIG. 2 can be visualized by reference to the corresponding waveform shown in FIG. 3. The time base for this system is provided by an astable, or freerunning multivibrator 30. The output of the astable multivibrator 30 is shown as curve a in FIG. 3. The output of the astable multivibrator 30 goes to a sweep generator 31. It is also sent to phase detector 32 to provide a reference input. The output from astable multivibrator 30 provides a trigger which starts and stops the sweep generator 31. Sweep generator 31 provides a linear sweep lasting over the entire period of the timing multivibrator 30. The output of the sweep generator 31 is shown as curve b in FIG. 3. The duration of the sweep 1s T seconds and the scanning repetition frequency (F) is cycles per second. The output of sweep generator 31 is sent in parallel to the input terminals of both vidicon tubes at terminals 26 and 27. Thus, the two vidicon tubes are swept in synchronism and as a result, they examine the field of vision looking at corresponding physical points at corresponding times.

When the sweep voltage is introduced in the vidicon tubes 17' and 18', they produce nooutput until such time when the scanning voltage is suchthat the portion of the face of the vidicon tube is scanned, which has an image formed by one of the two target spots 14 or 15 on surface 12A. When the vidicon tube as it is swept encounters an image on the face of the vidicon tube during the sweep, an output is produced lasting as long as there Also tube 18' is denoted as the A2 tube.

is a bright spot on the face of the tube, while the sweep voltage passes over this spot. Curves c and d in FIG. 3 show the respective outputs of vidicon tubes 17' and 18. For convenience the tube 37 is denoted the A1 tube because it is responsive to radiation of wavelength A1 only. Note that there is an output first from vidicon tube 17. This corresponds to a distance X between support 1 and surface 12A which is less than value h Thus there is produced an output first of vidicon tube 17' of scanner 23 and then, some time later, vidicon tube 18' produces an output corresponding to reflections from spot 15C in surface 12C. As has been pointed out it is important to determine which receiver produces the first output. The circuitry is arranged to do this. The output from vidicon tube 17' at terminal 28 is sent into a differentiating circuit 33. Likewise, the

output of vidicon tube 18' .at terminal 29 is sent in to another diflerentiating circuit 34. The output of differentiator 33 is shown as curve e in FIG. 3 and the output of diflerentiator 34 is shown as curve 1. The ditfer'entiator circuits 33 and 34 perform two functions. They provide a sharp output obtained from the leading edges of the pulse Outputs and d of vidicon tubes 17' and 18'. By diflerentiating the pulse outputs, the leading edges of the two pulses are examined. Also the difl'erentiator circuits clip off the negative portions of the waves so that only the leading edge of the output 0 and d in FIG. 3 are examined by the succeeding circuitry. It was mentioned in connection with FIG. 1, that it is only necessary that I the leading edges of the beams and 8 be sharply defined.

The actual width of thebeams is of little importance.

This is because the difierentiator circuits 33 and 34- only allows an output responsive to the leading edges ofthe pulses c and d. The actual area covered by the spots nected to the set input 40 of the A2 flip flop 39.

the waveforms of FIG. 3. Scanning may be performed at any convenient rate, for example, 1 to 100 kilocycles per second. The scanning rate will determine partly thedegree of precision of the measurement. The scanning ,rate'can be selected to provide the convenient correspending scale between time measurement and the distance measurement of X. For example in a radar type altimeter the speed of propagation of light determines the accuracy to which time measurements must be made. In

a pulse radar type altimeter the scale is 492 feet of distance per microsecond as determinedby the speed of light.

Hence in a radar type distance apparatus a one microsecond error in measuring time wouldproduce 492 feet of I T =1 millisecond 0 second With i=1 kc. a scale of one foot per microsecond can be i readily provided since in one millisecond the scanner could be arranged to scan 1000 linear feet by any of the geometrical arrangements shown. Hence a measurement error of one microsecond in the present invention would produce only one foot of error in the measured distance. Slower or faster scanning rates can be chosen depending on the accuracy desired, thedistance to be'scanned, and the precision to which time will be measured.

Reference to FIG. 2 will showthat the output of differentiator 33 is coupled to the,set input 35 of the A1 flipflop 36. Flip-flop 36 is denoted as the All flip-flop because it is responding basically to radiation of wavelengthAl from the projector 10. The output of difierentiator 33 is also coupled into a delay circuit 37 also, and the output of delay element 37 is coupled to the reset input 38 of flip-flop A2 denoted as 39. Thus, the output of diflerentiator 33 shown as curve 6 in FIG. 3 goes to the set input of A1 flip flop- 36 and to the reset terminal 38 of A2 flip flop 39 after a short delay provided by element 37. In a similar-manner, the output of differentiator 34 is con- Also the output of diiferentiator 34 is connected first to'delay ele ment '41 and then to the reset input 42 of the A1 flip flop 36. Thus, the differentiated outputs e and fot the vidicon tubes 17' and 18 are cross connected-to the two flip flops 36 and 39. V

The term set input is defined as follows. When a positive pulse is introduced into a set input the flip flop changes into a first state, the set state. The reset input is defined as follows. When a positive pulse is introduced into the reset input of a flip flop, the flip flop changes into the opposite state, 180 out of phase from the set state; this is the reset state. This is illustrated by curves g and h in FIG. 3. Curves g and It represent the outputs of the flips flops 36 and 39 in response to the input waves 2 and f of FIG. 3. Thus, when the positive-difierentiated pulse e enters the set input of flip flop 36, flip flop 36 changes from the reset condition of the flip flop with low voltage output to the set-condition of the flop, the

.36 and flip flop 39 together so that they are always exactly 180 degrees out of phase. flip flop 39 goes negative and vice versa.

When flip flop 36 goes positive,

the reset state, as shown by curve h. A very small amount of delay introduced by element 37 is not shown in curve h. The purpose of the delay elements 37 and 41' will be explained below. 7

At .some later ,time, the differentiated pulse 1 output from diflerentiator 34, causes flip flop 39 to go into the set state. This same '1 pulse, causes the flip flop 36 to go into the reset state. The outputs of flip flops 36 and 39 waveforms g and h are always out of phase with each other because of the cross connections of the set and reset inputs to the flip flops. The output of flip flop 36 is fed into input 43 of difference amplifier. 44. The output of flip flop 39 is put into the other input to the difference amplifier 4d. The diflerence amplifier circuit 44 has the propcity. that it takes the diflerence between its two inputs 43 and 45 and produces this difference as the output signal at its output terminal 46. Difference amplifier 44 takes the difference: (plus terminal 43minus terminal 45) shownby curve in FIG. 3. If the phase of the inputs at 43 and 45 reverses, the outputof the difference amplifier at 46 will also reverse in phase. The output of the difler ence amplifier is put into the second input to the phase detector 32. As mentioned before the first input to the phase detector is the basic timing waveform provided by the astable multivibrator 31). It can be seen that there is a diflerence in phase shown by between the output'of the difference amplifier at 46 and the reference input provided by the astable rnultivibrator 30. The output at 47 of the phase detector provides a voltage in proportion to this phase difference FIG. 4 shows the input-output characteristics of the phase detector. Thus, the ordinate shows the positive or negative voltage providedby'the output of the phase detector. The abscissa is shown in terms of distance or phase angle 5. The phase diflerence gb between the output of the difierence amplifier and the basic timing reference provided by circuit 30 directly corresponds to the distance X as measured from the crossover point at distance h The crossover point in the phase detector 32 is actually the distance I1 at which the beams 5 and 8 intersect in one common area. This is shown on FIG. 4 as the beam intersection distance where the output of the phase detector 32 is zero volts. Note that the use of two distinct wavelengths and the geometry of FIG.

Thus, the posi l I 1 have provided for this change in phase for distances above and below a reference distance h The output of the phase detector 47 is sent through a low pass filter 48 to remove any ripple and to provide a :smoothly varying output waveform. The output of the low pass filter 49 is actually the smooth output of the phase detector 47; This is a voltage as shown in FIG. 4,

which increases in magnitude as the distance away from the crossover value increases, and the polarity of the voltage indicates whetherthe distance is less than the distance 11 the crossover point or more than the distance h Thus, the plus axis of FIG. 4 corresponds to an A2 pulse being detected first. The negative polarity in FIG. 4 is shown for the condition of the first output corresponding to the A1 wavelength and the largest negative value of voltage corresponds to the lowest value of distance X which can be measured. As distance X increases, the output of low pass filter 49 moves up the curve of FIG. 4 towards the crossover point at k At the crossover point the distance is k X equals distance k and the output of the low pass filter is zero. As distance X increases beyond h to such a value as h for example, the output voltage further increases. Thus, a voltage is provided at 49, the output of low pass filter 48, which is a direct analog of the distance X between the support of the apparatus 1 and the surface 12A. This distance is an error voltage and is actually the distance k minus X, and with sense as shown. The error voltage 49 is fed to a distance indicator 50. This distance indicator 50 could take many forms, one of which would be a DC. voltmeter whose scale would be calibrated in terms of distance X. Another example would be a voltmeter with digital readout, so that the distance X would be displayed on separate dials as a digital number. A number of other possible arrangements could be used to indicate the distance X for display purposes for an operator.

But the error voltage 49 can be taken out and used directly as a control voltage rather than an indicating voltage or it may be'used as both. For example, FIG. 5 shows a scheme for using the present device in a closed loop servo control system for controlling the altitude of an aircraft. For example, low flying drones or missiles which attempt to hug the ground could use the present invention to maintain a constant altitude above the surface of the earth to avoid detection by enemy radar and for other tactical purposes. FIG. 5 shows the use of the invention of FIGS. 1 and 2 in a closed loop servo system for controlling the altitude of an aircraft. The aircraft might be a missile or a drone and the apparatus in this invention is particularly suited for holding the aircraft at rel atively low altitudes above the ground. The apparatus shown within the dashed lines in FIG. 5 indicated as 51, includes all the apparatus shown in FIGS. 1 and 2 operating in the manner described above. The system will also provide an indication of the altitude, if desired. An altitude indicator 50 is shown within block 51. It is similar to the distance indicator 50 in FIG. 2. The block labeled altimeter, numbered 52, is all of the equipment shown in FIGS. 1 and 2 with the exception of block 50. Thus, the output of block 51 in FIG. 5 is terminal 49 producing an error voltage to control the altitude of the aircraft. The input toblock 51 is indicated-at'53 as reflections. These are actually the reflections of the radiation from projectors and 11 striking the ground and being scanned by scanners 23 and 24 in the apparatus within block 51. Thus, the input 53 is actually a measurement of the altitude of the aircraft.

The error voltage from terminal 49 is shown as being introduced into a comparator 54 that takes the difference between its two inputs. The second reference, input to the comparator 54 is shown as the output voltage taken from the wiper 55 of a potentiometer 56. The position of the wiper 55 of potentiometer 56 is controlled .by a knob 57 and shaft 58 or other suitable means. The two ends of the potentiometer 56 are returned to power supply I12 voltages indicated as B+ and B- providing a positive voltage on one end and an equal negative voltage on the other end. There is a position near the middle of the potentiometer 56 where the output of the wiper 55 will be zero volts. The reason for providing the potentiometer and power supplies as an adjustable reference voltage is that the device of FIGS. 1 and 2 as shown in FIG. 5 will inherently produce a zero error voltage when the altitude is at height I2 as shown in FIG. 1. This is the inherent zero or servoing point of FIG. 1. It may be desired to hold the aircraft steady at a different altitude than 11 In that event, potentiometer 56 and wiper 55 are moved to a voltage corresponding to whatever particular altitude it is desired to hold the aircraft at. Knob 57 can be calibrated in terms of feet of altitude so that the aircraft can be adjusted to servo to any particular altitude. When the output of wiper 55 is zero volts, the system will inherently servo to the altitude k as shown in FIG. 1 since this is the zero error position as indicated in FIG. 4 as the output of phase detector 32 and low pass filter 48. This output is produced at terminal 49 as shown in FIGS. 2 and 5. The difference between the reference voltage 55 and the altitude error voltage 49 produces a servo error voltage E at the output of comparator 54 as shown in FIG. 5. This is an instantaneous error voltage E which will tend to correct the motion of the aircraft. The aircraft with its associated power and control servo systems is shown as a single block. This would consist of the aircraft itself along with the control surfaces of the aircraft such as, the ailerons, the rudder, flaps, etc. The error voltage E fed to the input of the power servos of these control surfaces will produce corrective motion vertically by the aircraft to bring the aircraft back to the correct altitude if it starts to deviate from this altitude. The output of the aircraft is the aircraft altitude and this aircraft altitude forms the input to the feedback altimeter system shown as block 51, This input is indicated at 53 as the reflections as just discussed.

It should be pointed out that although we have shown as an example a closed loop servo system involving an aircraft, this distance measuring apparatus can be used in many other types of closed loop servo mechanism systerns, such as, machine tools; on railroads to maintain a set distance between moving cars; on submarines to maintain a set difference below the surface of the water; and so on. It may also be pointed out if the distance indicating voltage or error voltage shown at terminal 49 in FIG. 2 is differentiated, it will provide the velocity of the surface 12 with respect to support 1. As shown, it indicates the instantaneous distance. Differentiating distance, we Would get velocity. This might be useful in certain applications. Reference to FIGS. 1 and 2 will show that the scanning of field of vision of the scanners 23 and 24 has been accomplished without mechanical motion of any of the component parts of this apparatus. However, the scanning motion can be accomplished by physical motion of the scanners 23 and 24. In another embodiment of this invention we will in fact illustrate such an arrangement.

A second embodiment of the invention is shown in FIG. 6A. For illustrative purposes, FIG. 6A shows the apparatus mounted on the underside of an aircraft 59 and the device is used as an altimeter. There are many other applications of the device, as have been discussed before. The scheme of FIG. 6A involves the use of two projectors, 6t) and 61, and one scanner 62. In this case, projectors 60 and 61 can be exact duplicates of each other and theymay produce radiation of'the same wavelength or different wavelengths, the only restriction being that scanner 62 responds to the wavelength from both projectors 6t) and el. Projector dti includes a source 68 of electromagnetic radiation similar to source 2 which may emit visible, ultraviolet, or infrared radiation and the same remarks concerning source 2 that were made under the discussion of FIG. 1 are equally applicable here. Pronarrower, that is, each intersection is narrower.

accomplished.

it produce a reasonably well defined beam of radiation 65 which has a predetermined angle F between the leading edge 66 and the trailingedge d7 of beam 65; o The angle of beam width Fmay take on a number of values depending 'upon the desiredjresults. The effect of the beam Width angle F upon the performance of the embodiment will be explained below. Also, the beam 65 of projector 6t! is inclined at an angle B from the horizontal, as shown in-.FIG. 6A. Projector 61' may besirnilar in every respect to projector 6t) and projector 61 is constructed in a similar manner including a source of electromagnetic radiation 69 and a lens system 70. Projector 61 produces a beam of radiation 71with leading edge 72 and trailing edge 73. The angle of beam width R between the leading 72 and the trailing edge 73 of beam 71 can assume any of a predetermined set of values. ,However, beam width angle R is not necessarily equal to the beam width angle F of projector 69. Beam 71 of projector 61 is inclined at an angle C fromv the horizontal as shown.

The effects of the values of the beam width angles F and R and the'values of angles B a'nd'C will be explained in the discussion-below. i

Projectors 6t and 61 are separated from each other by a distance D, as shown. The value ofD may be chosen appropriately depending upon the range of distances or altitudes to be measured. A larger value of D being somewhat more' appropriateto measure larger values of altitude or distance. In FIG. 6A the beams 65 and 71 are shown as converging but these beams mayv also be arranged indiverging relationship. 1 The beams may not bearrangedparallelato each other. The intersection of beam 65 from projector 60, with the surface or ground 77,is shown at 75 and the intersection of beam 71 from the second projector 51 is shown "as76. For purposes of- .sponding to ground level A. Curve 2B shows pulses corresponding to ground level B in FIG. 6A and so on. The

manner in which the curves shown in FIG. 6B is generated from the arrangement of FIG. 6A will now be explained. It will be noted that at the greatest altitude shown withground level at A, the intersections 75 and 76 of the beams 65 and 71 withthe ground 77 has a broad value, that is, the width of the intersection is quite large. Nowassume that-the aircraft 59 moved to a lower altitude suchthat the ground level is at Now the intersections 75B and 76B withthe ground 77B are In addition, the two intersections 75B and 76B are more widely separatedwith respect to each other. As the aircraft moves to a still lower altitude, such as shown at ground level C, the intersections 75C and 76C are still narrower and they are still further "separated from each other. If pulses were generated it can be seen that for any'particular altitude there is a unique value of the width of the intersections 75 and 76 and there is also a unique value of the separation between the intersections 75 and 76. By the measurement of the width-and/or the separation between these intersections of the'beams with the ground, the'measurement of altitude or distance may be Scanner 62 repetitively examinesflthe ground examining successive portions inoving'from the left to the right, as shown by the arrow labeled direction of scan. When scanner 62 encounters radiation reflected from the intersection of beam 65 with the ground at 75, output 78 is produced from scanner 62. As the scanner continues to This lens examine the ground 77, its output will decrease where it encounters the trailing edge 67 of beam 65. When the scanner 62 encounters the intersection 76 of beam 71 with the ground 77, another output is produced and this output ends when the scanner 62 encounters the trailing edge 73 of the beam 71. FIG. 63 illustrates the output of the scanner 62 for the various ground levels and is a projectionof the width and separation of the intersections 75 and 76 on to a time axis with voltage as the ordinate. The time axis is created by the uniform speed of scan of scanner 62 as it examines the ground plane 77. Thus, the time axis is an analog of distance measured along the surface 77 and voltage represents the output 78 of scanner 62. Examination of FIG. 6B will show that as the altitude decreases, the average value of the pulses produced by scanner 62 also decreases so that a direct analog. between the average or'D.C. value of the pulses in FIG. 6B and the distance of the apparatus from the surface 77 has been created. Utilization means 79 connected to the output 78 of scanner 62 measures the average value of the output 78 and provides an indication of altitude for the operator of the aircraft or provides 7 other useful signals in accordance with the altitude so measured. Thus, measurement of the average value of the output of scanner 62 provides a measure of the altitude or separation between the apparatus mounted on aircraft 59 and the ground 77. It can be readily seen that the same apparatus can be adapted to measure the distances' in many other types of applications, such as, machine tools; guiding electric buses through tunnels; and also to measure the height above ground of fog or cloud banks. Scanner 62 need not be in the same plane as the projectors 6t? and 61. Also, projectors 6t) and 61 need not be located on the same level with each other so long as the distance D between them is known and the orientation anglesB and C of the projectors from the horizontal is known.

FIG. 7A illustrates an arrangement of equipment and electronic circuitry for operation in accordance with the embodiment shown in FIG. 6A.

Projectors 6% and 61 are shown with their beams and 71 directed towards theground plane 77 as in FIG. 6A. The scanner s2 is shown in more detail. The lens system 81 focuses the reflections from the intersections and 76 with the ground 77 upon the face ofthe receiver 80. Receiver as shown here is a vidicon cameratype TV tube. The image orthicon-type tube could also :be used for element 86 with equal facility as discussed in connection with FIG. 2 of the first embodiment. One sweep terminal of vidicon tube 80 is shown at 26. Most vidicon tubeshave two sweep terminals, .one for horizontal defiections'and one for vertical deflections but as pointed out before, the present device scans in only one direction so that a simpler .type'of tube may be used with only one scan direction available or alternately a standard vidicon tube may be used and only one. of the scanning inputs used. Thus, terminal 26 can represent either the horizontal or vertical scan terminal. The output of vidicon tube 80 is indicated at terminal 28 and the vidicon tube produces an output when the scanning electron beam produced by the input to terminal 26 encounters an area of brighter illumination uponthe face of the vidicon tube. The small letters a, b, 0, etc. of FIG. 7A relate the output waveform shown in FIG. 8 to the various circuits of FIG. 7A. FIG. 8 is a timing diagram show- I ing the various waveforms of the circuits of FIG. 7A in time relationship to each other. The waveform e of PEG. 8 is the pulse waveform corresponding to waveform eA as shown in FIG. 6B, when the ground level is at A. The operation of block diagram of FIG. 7A

working according to the physical arrangement shown in FIG. 6A and FIG. 613 can now be explained. An astable multivibrator circuit 36 produces the basic time base waveform for the system. Curve a of FIG. 8 shows the output of the astable multivib-rator 30. The output of 15 multivibrator 30 is connected to the input of sweep generator 31. Sweep generator 31 is started and stopped by the timing waveform from astable multivibrator 30, as shown in curve b of FIG. 8. The output of sweep generator 31 is connected to one of the sweep terminals 26 of the vidicon tube 8t) and provides the sweep for the scanning electron beam of the vidicon tube 89. Thus, the circuitry provides a correspondence between the time in each scanning cycle and the linear distance shown in the field of the scanner 62.. Astable multivibrator 30 may run at any convenient scanning rate. A typical scanning rate might be kilocycles per second, but slower or faster scanning rates may be used as desired for less or more accuracy. A slower scanning rate lowers the accuracy requirements on the circuitry and allows larger errors in time measurement to correspond to smaller errors in altitude or distance measurement. The output terminal 28 of vidicon tube 30 is connected to diiferentiator circuit 33. Curve c of FIG. 8 shows the output of the vidicon tube 80. It will also be seen that the output of the vidicon tube 80 corresponds to curve eA of FIG. 6B. In theory, the output of vidicon tube 80 can be connected directly to an average value detecting circuit such as 82 in FIG. 7A. The device will actually work well this way and it will provide extreme simplicity in circuitry. However, the output of vidicon tube 80 is first put through differentiator circuit 33 and then the output of differentiator circuit 33 shown as waveform d is connected to the input 83 of fiip flop circuit 84. Difierentiator circuit 33 responds to both the positive going leading edge of the output of vidicon tube 80 and to the negative going trailing edge of the output of vidicon tube 80 as shown in curve d of FIG. 8. The ditferentiator circuit insures that there will be a sharp pulse indication of the beginning of the intersection 75 or 76 correspondingto leading edges 66 and 72 of beams 65 and 71. The output of ditferentiator circuit 33 is connected to input 83 of bistable multivibrator 84 so that bistable multivibrator 84 is triggered into the set state by positive going pulses and into the reset state by negative going pulses, one input A accepting both positive and negative pulses- Flip-flop multivibrator 84- acts as both an amplifier of the output of differentiator 33 as a peak clipper so that positive or negative pulses of a uniform magnitude are produced. There'will, then, be no shift in the scale of voltage since in this apparatus the average value of the voltage output of flip flop 84 corresponds to the distance or altitude to be measured. The output of multivibrator 84 is shown as curve e in FIG. 8. The curves in FIG. 6B are shown as only positive going pulses. This is because the average value detect-or 82 clips off the negative portion of the waves so that there will always be present a net average value of DC. voltage out of the flip flop 84. The output of flip flop 34 is connected to the input of average value detector 82. Average value detector 82 produces an output voltage corresponding to the average position value of the output from flip flop 84. The output of average value detector 82 is shown at terminal 85 as an error voltage. The value of this error voltage is a direct analog for the distance between the apparatus mounted in aircraft 59 and the ground 77 or from the apparatus to whatever surface it is desired to measure. Output terminal 85 is connected to an altitude indicator 86. Altitude indicator 86 may be one of several types well known in the art, such as, a DC. voltmeter calibrated in terms of altitude in feet or a voltmeter with a digital readout, etc. To illustrate the extreme simplicity of the circuitry involved in FIG. 7A, FIG. 7B shows a detailed circuit which is one of many possible forms for the average value detector 82. As shown in FIG. 7B, input voltage e is coupled to the detector 82 by coupling capacitor 87, with resistor 88 providing the ground return for capacitor 87. Diode 89 clips olf the negative portion of the output of flip flop 84 as discussed above. Resistor 99 in conjunction with capacitor 911 provide a low pass RC circuit with a operation.

relatively long time constant which responds to the average value of the input wave 2. The output is at terminal $5 which is the junction of resistor 90 and capacitor 91. Increasing positive voltages indicate larger distances and zero voltage would indicate the minimum altitude which can be measured by the apparatus.

It will be readily understood that the embodiments of FIGS. 6A, and 7A can be used in a closed loop control system such as shown in FIG. 5, as discussed in connection with the embodiments of FIG. 1. When the apparatus of FIG. 6A and FIG. 7A is used in a closed loop control system, the output terminal of the distance measuring apparatus would be terminal instead of terminal 49. Again, the input to the apparatus in block 51 will be the reflections from the ground surface 77 as shown at 53 in FIG. 5. There is only one minor difierence in The embodiments of FIGS. 1, 2 and 5 inherently servo about a particular zero signal altitude such as I1 where the two beams intersect. The embodiment of FIG. 6A has no such inherent reference altitude, but for any particular altitude, FIG. 6A readily provides a distinctive voltage corresponding to that altitude. It is only necessary when using the embodiments of FIGS. 6A and7A in a closed loop servo mechanism system such as described in FIG. 5 to set the wiper of potentiometer 56, for example, to the voltage corresponding to the altitude which it is desired to maintain. Then, the apparatus of FIGS. 6A and 7A will produce this same voltage at the correct altitude. The difference taker 54 will subtract the two equal voltages, one from output terminal 85 of the altimeter and the other from the wiper 55 of potentiometer 56. The error voltage shown as E in FIG. 5 will be zero when the aircraft is at the correct altitude. It can be readily seen that by adjustment of potentiometer 56 the equipment of FIGS. 6A and 7A may be made to keep the aircraft at any desired altitude within the range of the instrument. The circuitry described in FIG. 7A

may be implemented by many technique well known in the art and may be constructed using transistors, vacuum tubes or magnetic cores or other active elements. It will be appreciated that the embodiment of FIG. 7A can be built using less than ten transistors, for example, so that the present device provides extremely simple electronic circuitry for accomplishing the measurement of distance. FIG. 9 shows the response characteristics required of the average value detector 32. The abscissais plotted as the distance or altitude scale, and the output voltage of average value detector 82 is the ordinate. The output curve starts at point 92 corresponding to the lowest altitude, which can be of convenient value according to the adjustments of the parameters described in FIG. 6A. An altitude is shown at point 93 in FIG. 9 corresponding to the largest altitude that can be measured.

It is desirable to confine the range of the apparatus to the linear range of the average value detector 82. Other types of average value detector may be readily substituted in place of the circuitry shown in FIG. 7B. It will be noted that the response curve of FIG. 9 has a particular slope In other words, for a given change in altitude there will be a corresponding change in the voltage output of average value detector 82. This slope may be readily varied in two ways. If the beam width angles F and R, shown in FIG. 6A, are made larger the slope of FIG. 9 will become steeper so that for a given change in altitude there will be a larger corresponding change in the output voltage of the average value detector 82 and vice versa. The second technique to alter the slope of the response curve of FIG. 9 is by means of the angles B and C of FIG. 6A.' Thus, if the separation between the intersections 75 and 76 is made larger by increasing angles B and C in FIG. 6A, the average value detector 82 of FIG. 7A will produce a somewhat smaller voltage be- 

1. A SYSTEM FOR MEASURING THE DISTANCE TO A SURFACE COMPRISING AT LEAST ONE SOURCE OF ELECTROMAGNETIC RADIATION, AT LEAST ONE RECEIVER RESPONSIVE TO SAID RADIATION REFLECTED FROM SAID SURFACE, MEANS DISPOSING SAID SOURCE AND SAID RECEIVER IN PREDETERMINED GEOMETRICAL RELATION WITH RESPECT TO EACH OTHER AND SAID SURFACE, SCANNING MEANS COUPLED TO SAID RECEIVER, MEANS TO SECURE FROM THE OUTPU TOF SAID RECEIVER TWO DISTINCTIVE DIGITAL OUTPUTS REPRESENTING VARIATIONS IN DISTANCE BETWEEN SAID SOURCE AND SAID SURFACE IN ACCORDANCE WITH SAID GEOMETRICAL RELATION AND MEANS RESPONSIVE TO SAID DIGITAL OUTPUT SECURING MEANS TO COMPARE SAID OUTPUTS AND PRODUCE ANOTHER OUTPUT AND MEANS RESPONSIVE TO SAID COMPARISON OUTPUT TO INDICATE SAID DISTANCE. 